Radiotherapy is one of the two most effective treatments for cancer. The success of radiotherapy in curing cancer depends critically on accurate targeting and delivery of the correct radiation dose. If the dose delivered to a patient is too low then cancerous cells may survive leading to a recurrence of the cancer. If the dose delivered is too high then surrounding healthy tissue is more likely to be damaged. For example, optimal treatment of some head and neck tumours requires that the dose delivered should be within only a few percent of that prescribed. Uncertainty in patient positioning means that it is crucial for all other errors to be as small as is possible. Accurate dosimetry is therefore essential to maintain and improve patient survival rates.
Radiation dosimetry is the measurement and calculation of the absorbed dose in matter and tissue resulting from the exposure to indirect and direct ionizing radiation. It is a scientific subspecialty in the fields of health physics and medical physics that is focused on the calculation of internal (internal dosimetry) and external doses from ionizing radiation. In medical physics absorbed dose is reported in SI units of gray (Gy) where 1 Gy=1 J/kg, and in radiation protection dosimeters in units of Sieverts (Sv).
There are different ways of measuring absorbed dose from ionizing radiation. For workers who come in contact with radioactive substances or may be exposed to radiation routinely, personal dosimeters are typically employed and intended primarily for warning/notification rather than accurate determination of dose. In the United States, these dosimeters are usually thermoluminescent dosimeters (TLD) or optically stimulated luminescence (OSL) dosimeters, whilst personal dose monitors based on photographic emulsions that are sensitive to ionizing radiation are also available. In radiotherapy, such as with linear particle accelerators in external beam radiotherapy, routine accurate calibration is typically and most commonly obtained using ionization chambers. However other detectors ranging from semiconductor-based dosimeters to radiochromic films may also be used for certain applications.
Because the human body is approximately 70% water and has an overall density close to 1 g/cm3, for consistency, absorbed dose measurements are normally made in and/or reported as dose to water. National standards laboratories such as US National Institute of Standards and Technology (NIST) and UK National Physical Laboratory (NPL) provide calibration factors for ionization chambers and other measurement devices that are used to convert the instrument's readout, which may be for example ionization, optical density change, current, etc., to absorbed dose to water. The standards laboratories maintain a primary standard, which is normally based on either of three techniques: calorimetry, Fricke dosimetry, or free air ionization chambers. Out of the three, calorimetry, being the measurement of temperature rises due to radiation energy being absorbed in medium, is the most direct and absolute means of determining absorbed dose and is used most commonly.
A hospital or other users subsequently send their detectors (often ionization chambers) to the laboratory, where it is exposed to a known amount of radiation (as determined using the primary standard) and in turn a calibration factor is issued to convert the instrument's reading to absorbed dose. The user may then use this calibrated detector (secondary standard) to derive calibration factors for other instruments they use (tertiary standards) or field instruments. The uncertainty on the calibration factor of a detector increases inherently with the number of steps in the chain of calibrations relating the device to the primary standard.
Today many primary standards laboratories use water- or graphite-calorimeters to maintain an absolute photon dosimetry standard. In calorimetry, the basic assumption is that all (or a known fraction) of the absorbed radiation energy appears as heat, so that the measurement of absorbed dose reduces to a measurement of a temperature change. If the absorbed dose to water is to be established, ideally the calorimetric measurements should be made using water, see for example Ross et al in “Water calorimetry for Radiation Dosimetry” (Phys. Med. Biol., Vol. 41, pp 1-29). However, due to many challenges with water calorimetry, including low signal to noise ratio and potential heat defect due to presence of impurities in water, in addition to the cumbersome nature of the device and difficulty of working with a water tank and related accessories, significant research has also been undertaken in the area of graphite calorimetry. Graphite has beneficial radiation absorption characteristics that are similar to those of water, and allows for thermally isolated segments to be machined and configured so as to permit the measurement of absorbed dose to graphite. At present due to advances in water calorimeters arising from the work of Domen, see for example “Absorbed Dose Water calorimeter” (Med. Phys., Vol. 7, pp 157-159), both graphite and water calorimeters are exploited.
However, due to their general bulkiness and long wait times with establishing thermal equilibrium water and graphite based calorimeters have been to date, within the prior art, limited to standards laboratories. Accordingly, it would be beneficial to provide clinical medical physicists with an alternative approach to ionization chambers for the calibration and quality assurance of radiation therapy equipment including standard as well as small radiation fields. It would be further beneficial for such novel clinical dosimeters to be capable of operating as self-calibrating secondary standards, which may be used routinely for measurements rather than calibration activities only.
Radiotherapy is a field subject to continuing evolution as treatment protocols, radiopharmaceuticals, and radiotherapy equipment address both the rising rates of cancer, as more people live to an old age and as mass lifestyle changes occur in the developing world such that in 2007 approximately 13% of all human deaths worldwide (7.9 million) were cancer related, and currently there are over 200 different known types of cancers. Amongst such developments is the emergence of treatment units specifically designed for stereotactic radiosurgery, wherein small targets inside the body are treated using small static or rotating radiation fields that are at times highly modulated in both intensity and/or shape. Many radiotherapy units such as GammaKnife®, CyberKnife®, TomoTherapy®, and even most conventional linear accelerator (LINAC) manufacturers Varian®, Siemens®, Elekta® now provide the capabilities of delivering extremely complex treatment deliveries based on stereotactic radiotherapy (SRT) or intensity modulated radiation therapy (IMRT) to treat a given disease site with extreme accuracy and conformality. Accordingly, with these sophisticated techniques comes the requirement for new dosimetry protocols that address absorbed dose calibration in nonstandard radiation fields wherein practices are currently lacking international standards, see for example Alfonso et al in “A New Formalism for Reference Dosimetry of Small and Non-Standard Fields” (Med. Phys., Vol. 35, pp 5179-5186). Accordingly, calorimetry could offer a more direct and accurate way of measuring absorbed dose to water in small and composite radiation fields by doing away with the need to transfer calibration factors according to the radiation beam quality of interest.
However, calorimetry is not without its challenges when considering compact field-deployable calorimeters. Graphite has a specific heat capacity one-sixth that of water and therefore for a given radiation dose, its temperature rises 6 times more than an equivalent water based calorimeter give rise to a higher signal to noise ratio. A typical dose of radiation to a human during radiotherapy treatment is approximately 1-2 Gy, which is 1-2 joules per kilogram. Accordingly, if we consider a calorimeter comprising a 1 cm3 piece of graphite, which weighs approximately 2 grams, this would therefore absorb around 2-4 mJ, which with a specific heat capacity of around 700 Jkg−1 K−1 equates to a temperature rise of just 1-2 mK. Accordingly, significant problems exist in insulating the graphite from the ambient clinical environment in order to measure such tiny temperature changes.
Accordingly, embodiments of the invention provide for compact graphite probe calorimeters (GPC) for absolute accurate clinical dosimetry to address the requirements of evolving radiotherapy systems and radiopharmaceutical therapies whilst providing medical radiation oncology technologists and medical physicists with compact, fast, low cost alternatives to ionization chambers for conventional radiotherapy calibration.
Other aspects and features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.